1. Field of the Invention
The present invention relates to a tunnel magnetoresistive sensor that may be installed in a hard disk drive or be used as a magnetoresistive random access memory (MRAM), and more specifically, it relates to a tunnel magnetoresistive sensor that can achieve a high rate of resistance change (ΔR/R) when Mg—O is used as an insulating barrier layer, and to a method for manufacturing the tunnel magnetoresistive sensor.
2. Description of Related Art
Tunnel magnetoresistive sensors generate a resistance change by utilizing a tunneling effect. When the magnetization of a pinned magnetic layer is antiparallel to the magnetization of a free magnetic layer, less tunneling current flows through an insulating barrier layer (tunnel barrier layer) disposed between the pinned magnetic layer and the free magnetic layer, and thereby the resistance reaches its peak. On the other hand, when the magnetization of the pinned magnetic layer is parallel to the magnetization of the free magnetic layer, the tunneling current reaches the maximum, and the resistance reaches the minimum.
According to this principle, an external magnetic field changes the magnetization of the free magnetic layer and thereby changes the electrical resistance. The tunnel magnetoresistive sensors detect the change in electrical resistance as a voltage change and thereby detect a leakage field from a recording medium.
Important characteristics of the tunnel magnetoresistive sensors include the rate of resistance change (ΔR/R) and RA (resistance R×area A). The material for an insulating barrier layer, the materials for a pinned magnetic layer and a free magnetic layer disposed on both sides of the insulating barrier layer, and the structure of these layers have been improved to optimize these characteristics.
Japanese Unexamined Patent Application Publication No. 2004-179667 and No. 2005-197764 describe tunnel magnetoresistive sensors.
One task of tunnel magnetoresistive sensors is to achieve a high rate of resistance change (ΔR/R) to increase the detectivity and thereby improve the characteristics of a playback head. It is known that optimization of the composition of a free magnetic layer or a pinned magnetic layer and optimization of the crystal structure of an insulating barrier layer or the free magnetic layer, including use of a material having high spin polarizability at an interface with the insulating barrier layer, are important to increase the rate of resistance change (ΔR/R) of a tunnel magnetoresistive sensor.
Since different materials for the insulating barrier layer impart different characteristics including the rate of resistance change (ΔR/R), research must be conducted in a manner that depends on the material of the insulating barrier layer.
For example, in a structure composed of an antiferromagnetic layer, a pinned magnetic layer, an insulating barrier layer, and a free magnetic layer laminated in that order from the bottom, when the pinned magnetic layer has a layered ferri structure composed of a first pinned magnetic sublayer, a nonmagnetic intermediate sublayer, and a second pinned magnetic sublayer laminated in that order from the bottom, and when the insulating barrier layer is formed of Mg—O, the second pinned magnetic sublayer was heretofore formed of CoFeB.
CoFeB that contains a high concentration of B is known to be predominantly amorphous in a non-heat-treated (as deposited) state. Thus, when the second pinned magnetic sublayer is amorphous, the insulating barrier layer and the free magnetic layer formed on the second pinned magnetic sublayer is thought to be less affected by the crystalline orientation under the second pinned magnetic sublayer. This will increase the crystalline orientation and thereby increase the rate of resistance change (ΔR/R).
However, the second pinned magnetic sublayer crystallized incompletely from the amorphous state even when heat treated in a manufacturing process of the tunnel magnetoresistive sensor. Consequently, it turned out that, in the structure described above, the crystalline orientation was improved insufficiently, and the high rate of resistance change (ΔR/R) could not be achieved.
FIG. 12 shows the rate of resistance change (ΔR/R) as a function of the composition ratio Y of B in CoFeB constituting a second pinned magnetic sublayer in a layered body composed of an underlying layer; Ta (30)/seed layer; Ru (40)/antiferromagnetic layer; IrMn (70)/pinned magnetic layer [first pinned magnetic sublayer; Co70at % Fe30at % (22)/nonmagnetic intermediate sublayer; Ru (9.1)/second pinned magnetic sublayer; (Co50% Fe50%)100-YBY (20)]/insulating barrier layer; Mg—O (11)/free magnetic layer [enhance sublayer; Co50at % Fe50at % (10)/soft magnetic sublayer; Ni87at % Fe13at % (50)]/protective layer [Ru (20)/Ta (180)] laminated in that order from the bottom. The figures in parentheses are average film thicknesses expressed in angstroms. In this experiment, the layered body was annealed at 270° C. for 3.5 hours.
As shown in FIG. 12, when the composition ratio Y of B was approximately in the range of 15 to 20 atomic percent, the rate of resistance change (ΔR/R) could increase, but was still low. In addition, as shown in FIG. 12, the rate of resistance change (ΔR/R) changed greatly with the composition ratio Y of B. Thus, a high rate of resistance change (ΔR/R) could not consistently be achieved. Although a large composition ratio Y of B promotes amorphization in a non-heat-treated (as deposited) state, a high rate of resistance change (ΔR/R) is not achieved, as shown in FIG. 12, indicating that crystallization does not ocurr with heat treatment.
Japanese Unexamined Patent Application Publication No. 2004-179667 discloses a magnetoresistive sensor in which an enhance sublayer in contact with an insulating barrier layer has a thickness of 2 nm or less. While this patent document discloses an insulating barrier layer formed of Al—O or Mg—O, all the insulating barrier layers described in Examples are formed of alumina (Al—O).
Furthermore, the tunnel magnetoresistive sensor described in Japanese Unexamined Patent Application Publication No. 2005-197764 has a shielding layer formed of a microcrystalline material to minimize the surface roughness of the insulating barrier layer, thus achieving high output. Furthermore, this patent document discloses a pinned layer formed of CoFe or CoFeB, but does not disclose an insulating barrier layer formed of Mg—O.
Thus, both of the above-referenced patent documents do not relate to a tunnel magnetoresistive sensor including an insulating barrier layer formed of Mg—O. In these patent documents, the composition or the structure of a free magnetic layer or a pinned magnetic layer is optimized to improve the characteristics of a tunnel magnetoresistive sensor. Thus, these patent documents do not describe the optimization of the crystal structure of an insulating barrier layer, a free magnetic layer, or a pinned magnetic layer to increase the rate of resistance change (ΔR/R) of the tunnel magnetoresistive sensor.